Answer: Step 23 - Bob installs adapter plate in open slot 2.


The list of steps in the corresponding spacewalk mission:
Step 1 - Chris exits airlock.
Step 2 - Chris sets up tether.
Step 3 - Bob grabs foot restraint and follows outward.
Step 4 - Chris sets up foot restraint.
Step 5 - Chris heads to external pallete.
Step 6 - Chris sets up tools and prepares worksite.
Step 7 - Bob moves to IEA.
Step 8 - Chris & Bob finish setup.
Step 9 - Chris & Bob retrieve battery from slot 1.
Step 10 - Chris & Bob move battery to EP.
Step 11 - Chris enters foot restraint.
Step 12 - Chris & Bob install battery in empty slot X.
Step 13 - Chris & Bob remove new battery from slot A.
Step 14 - Chris & Bob move battery to truss.
Step 15 - Chris & Bob install empty battery in empty slot 1.
Step 16 - Bob move foot restraint to reach battery.
Step 17 - Chris & Bob retrieve battery from slot 2.
Step 18 - Chris & Bob move battery to EP.
Step 19 - Chris enters foot restraint & places battery on BRT.
Step 20 - Chris releases adapter plate.
Step 21 - Chris removes adapter plate and passes it to Bob.
Step 22 - Bob takes the adapter plate to the truss.
Step 23 - Bob installs adapter plate in open slot 2.
Step 24 - Chris puts old battery in slot A.
Step 25 - Chris & Bob make cable between new battery and adapter plate.
Step 26 - Chris & Bob remove old battery from slot 3.
Step 27 - Chris & Bob stow battery on adapter plate in slot 2.
Step 28 - Chris & Bob move to EP.
Step 29 - Chris removes new battery and hands it to Bob.
Step 30 - Chris & Bob take the battery to the truss.
Step 31 - Chris & Bob install new battery in empty slot 3.
Step 32 - Chris & Bob cleanup and setup for future EVAs.
Step 33 - Chris moves to airlock.
Step 34 - Bob moves to airlock.
Step 35 - Chris & Bob enter airlock.